This invention relates to a centralizer for a down hole tool for oil and gas well completion and servicing operations. More particularly, the invention relates to such a centralizer which is especially adapted for use in dual well completion applications.
During oil and gas well completion and servicing operations, the need often arises to run a well servicing tool from the rig down to a piece of equipment located in the wellhead assembly, for example, to install a back pressure valve (xe2x80x9cBPVxe2x80x9d) in the production bore or receptacle of a tubing hanger mounted in the wellhead. This is usually accomplished by attaching the tool to a running string, such as a sucker rod or drill string. In typical oil and gas wells, the wellhead may be located a substantial distance below the rig floor. In addition, a blow out preventer (xe2x80x9cBOPxe2x80x9d) stack is usually connected to the wellhead during completion and servicing operations to contain any inadvertent well blowouts that may occur. In offshore wells, the wellhead is connected to the rig by a riser pipe. Thus, during completion and servicing operations the servicing tool must be run down through the riser and the internal bore of the BOP stack, and the possibility exists that the tool may impact the riser or the BOP stack, causing damage to the BOP stack or the tool itself. Furthermore, if a blow out occurs while the tool is positioned in the wellhead, the tool may be propelled back into the BOP stack and damage the BOP rams.
The prior art has addressed this problem by providing a tool protection guide between the running string and the well servicing tool to center the tool within the riser and the BOP bore as the tool is being run into the wellhead. As disclosed more fully in U.S. Pat. No. 5,730,218, which is owned by the assignee hereof, this prior art tool protection guide comprises an elongated mandrel having a top end which is connected to the running string and a bottom end to which the tool is connected, a circular top collar having a concentric hole through which the mandrel extends, means for rotatably connecting the top collar to the mandrel, a circular bottom collar which is connected to the top collar by a number of bow springs which together form a centralizing shroud for the tool, and an energy absorbing bumper positioned above the top collar and having an opening in alignment with the hole in the top collar through which the top end of the mandrel projects. Thus, the centralizing shroud keeps the tool centered within the riser and the bore of the BOP stack as it is being run down to the wellhead to prevent the tool from impacting the riser or the BOP stack, and the energy absorbing bumper absorbs the impact on the BOP rams should the tool be propelled into the BOP stack during a well blow out. However, since the mandrel is centered within the centralizing shroud, this tool protection guide is not useful for running tools into dual completion wellheads, which typically comprise a dual tubing hanger having two production bores or receptacles which are offset from center. Thus, a need exists for a guide or centralizer which can align the well servicing tool with the offset bores of a dual completion wellhead.
The present invention addresses this and other problems by providing a centralizer for guiding a well servicing tool through the bore of a BOP stack or the like and into a radially eccentric position within a wellhead assembly, the centralizer comprising a mandrel having a top end which is connected to a running string and a bottom end to which the well servicing tool is connected, a collar having at least one eccentric hole through which the mandrel extends, means for rotatably connecting the collar to the mandrel, and an energy absorbing bumper supported above the collar and having at least one opening in alignment with the hole in the collar through which the top end of the mandrel projects. The distance between the center of the collar and the center of the eccentric hole is selected to correspond to the radial distance between the center of the BOP bore and the eccentric position within the wellhead assembly. In addition, in the preferred embodiment of the invention the outer diameters of both the collar and the bumper are selected to be only slightly smaller than the diameter of the BOP bore so that the collar and the bumper will maintain the mandrel, and thus the tool, in alignment with this eccentric position as the tool is being run into the wellhead. Furthermore, since the mandrel is rotatable with respect to the collar and the bumper, the centralizer may be used to run tools which require either linear or rotary action to be set in the wellhead component.
In an exemplary embodiment of the invention, the well servicing tool is a BPV and the eccentric position in the wellhead assembly corresponds to one of the production bores or receptacles of a dual tubing hanger in a dual completion wellhead. As the BPV is being run through the BOP stack, the collar and the bumper will maintain the BPV in alignment with the receptacle in the tubing hanger and prevent the BPV from contacting the BOP bore. In addition, if a blow out should occur and the BPV and running string are propelled back into the BOP stack, the energy absorbing bumper will absorb the energy of the impact between the BPV and the BOP rams and thereby prevent or minimize damage to the BOP rams.
These and other objects and advantages of the present invention will be made apparent from the following detailed description, with reference to the accompanying drawings. In the drawings, the same reference numbers are used to denote similar components in the various embodiments.